Over the years, Ethernet has become the most commonly used method for local area networking. The IEEE 802.3 group, the originator of the Ethernet standard, has developed an extension to the standard, known as IEEE 802.3af, that defines supplying power over Ethernet cabling. The IEEE 802.3af standard defines a Power over Ethernet (PoE) system that involves delivering power over unshielded twisted-pair wiring from Power Sourcing Equipment (PSE) to a Powered Device (PD) located at opposite sides of a link. Traditionally, network devices such as IP phones, wireless LAN access points, personal computers and Web cameras, have required two connections: one to a LAN and another to a power supply system. The PoE system eliminates the need for additional outlets and wiring to supply power to network devices. Instead, power is supplied over Ethernet cabling used for data transmission.
As defined in the IEEE 802.3af standard, PSE and PD are non-data entities allowing network devices to supply and draw power using the same generic cabling as is used for data transmission. A PSE is the equipment electrically specified at the point of the physical connection to the cabling, that provides the power to a link. A PSE is typically associated with an Ethernet switch, router, hub or other network switching equipment or midspan device. A PD is a device that is either drawing power or requesting power. PDs may be associated with such devices as digital IP telephones, wireless network access points, PDA or notebook computer docking stations, cell phone chargers and HVAC thermostats.
PSE's main functions are to search the link for a PD requesting power, optionally classify the PD, supply power to the link if a PD is detected, monitor the power on the link, and disconnect power when it is no longer requested or required. A PD participates in the PD detection procedure by presenting a valid or non-valid detection signature to request power and indicate that power has been received. The PD detection signature has electrical characteristics measured by the PSE.
Electrical signals can be carried over cables in a normal, differential or common mode. A normal-mode signal appears between a pair of wires or on a single wire referenced to (or returned via) the earth, chassis or shield. Normal-mode signals are read between two wires in a balanced or unbalanced transmission path. A differential-mode signal appears differentially on a pair of wires in an ungrounded cable configuration. A common-mode signal appears equally with respect to local common or ground on both lines of a 2-wire transmission system.
Traditional data transmission over Ethernet is carried out between the Ethernet twisted pairs in a differential mode, where one pair is used for transmitting data from a first node to a second node, and the other pair is used for receiving data sent from the second node to the first node. In a PoE system, power between a PSE and a PD is transmitted in a common mode as a voltage between two of the Ethernet twisted pairs, typically by powering the center-taps of the isolation transformers used to couple the Ethernet data signals to the wire. Since Ethernet data are sent differentially, the power transmitted in a common mode between the Ethernet transmit pairs and receive pairs does not affect the Ethernet data transmitted in a differential mode. A PD detection and powering protocol in a PoE system involves an analog scheme to sense impedance or current on the common-mode connection of the Ethernet twisted pairs.
Although PSE and PD are non-data entities, which are not involved in transmission of Ethernet data, it would be desirable in some cases to provide data transmission between the PSE and the PD, for example, for identification purposes.